Selenium coated elements and method of making them



Sept. 24; 1946. VQN 2,408,116,.

SELENIUN comma ELEMENTS AND METHOD or MAKING THE! Filed July 12, 1941sag/W0 2 J'LE/V/DE 1744/ IN VENTOR ATTORNEY PalentedWMlHB' SELENIUMCOATED ELEMENTS AND METHOD OF MAKING THEM Arthur von Hippel, Weston,Mass., assignor to Federal Telephone & Radio Corporation, a corporationof Delaware Application July 12, 1941, Serial No. 402,103

Claims. 1

This invention relates to processes of treatin metal surfaces andproducts thereof, and it comprises processes wherein metal surfaces aresubiect to the action of selenidingagents to form a film including aform of selenium, presumably a metal selenium compound on the surface;it further comprises processes wherein such coated surfaces arethereafter further coated with selenium, and it further comprises theproducts of such processes.

The application of selenium to metals, such as iron, nickel. and others,for the purpose of preparing photoelectric cells, rectifiers, and thelike, has hitherto presented many problems. Selenium metal iscustomarily applied by spreading the molten selenium on the metalsurface or otherwise producing a smooth coating on one surface.Selenimn, however, has a very high tension and it is difilcult to wetthe metal surface with the molten selenium. As a consequence theselenium layer is not uniformly spread on the disc and does not adherefirmly thereto. The problem involves at least two factors. The first isthat of so changing surface tension relationships between moltenselenium and the metal to be coated that the selenium will spread evenlyover the metal. This can be realized by thoroughly cleaning the metalsurface prior to applying the selenium. The second factor has to do withestablishing a firm bond of adhesion between the selenium and the metal.The adherence of the film to the metal is of great importance in platesor discs to be used as rectifiers or light sensitive cells, andhithertothere has been no satisfactory way of improving the "bond"between the metal and the selenium film.

It has been previously found that an improved bond could be obtained byroughing the surface of discs before applying selenium thereto. However,even this rougheni'n'g does not secure a sufilciently close or firmbond. Moreover. it cannot be satisfactorily applied to base metal discssuch as iron or aluminum discs which have been plated with anothermetal, such as nickel'or zinc, because the roughening treatment will cutthrough the plating exposing the plated metalto corrosion.

I have now discovered ways by which selenium matings on metal surfacescan be made to adhere tenaciously thereto and thus I am able to prepareselenium-coated metals which resist ro'ugh handling, abrasion, and thelike, so that the selenium plates are not readily damaged in use.

My invention is based upon the discovery that In the single figure ofthe drawing is shown. by way of example, a disc, partly broken away,which may be considered as a selenium rectifier disc or aphoto-sensitive element made in accordance with my invention. Thedimensions of the disc are exaggerated in order to more clearlyillustrate the component parts.

This comprises a metal disc I coated with a film 2 of a metal seleniumcompound of the metal. On the upper surface of the disc is shown aselenium layer 3.

Thus my invention in part can be looked upon as the formation of ametal-selenium film on the metal prior to the application of metallicselenium thereto with the metal "selenide acting as a bonding agent forbonding the metal to the overlying selenium coating. This film alsoimproves the surface tension relationship so that ent ways. One of themost convenient is that of selenium metal adheres much more strongly todipping the metal, or otherwise treating its surface, with aqueoussolutions of selenious acid or a. selenate, to which in some casesnitric acid has been added. For example, one suitable solution (forconvenience called bath 1) consists of an aqueous nitric acid solutioncontaining, in each liter, about 252 grams of nitric acid and about 105grams of selenious acid expressed as $802. Such a solution isapproximately 4 normal in nitric acid. The metal surface on which a filmof the selenide is to be formed is dipped or otherwise contacted withsuch a solution. The time of dipping is not important, from two to tenminutes is adequate. In such a solution the metal of the surfaceundergoing treatment reacts with selenium ions to form a metal-seleniumcompound.

Theoretically, the reactions occurring are probably as follows, usingnickel as an example.

When the nickel, for example, a strip or disc thereof, is immersed inthe solution, the surface of the nickel is attacked by the acid in thesolution and nickel ions are formed. A certain amount of nitrous oxide(NO) is also liberated and the nitrous oxide reduces selenious acidpresent to liberate divalent selenium ions. These negative ions reactwith the positive nickel ions to form nickel selenide, NiSe. However,the reactions actually occurring may be somewhat more complicated thanthis simple explanation would indicate. In any event, the nickelselenium compound is deposited on the surfaces of the nickel strip ordisc exposed to the solution. Then the treated metal is withdrawn fromthe aqueous solution. washed and dried. It can thereafter be coated withmolten selenium and the selenium layer thus provided on the metal isuniform throughout and adheres firmly to the base metal.

The temperature during the selenidin8" operation is not criticalalthough I find it advantageous to use moderately elevated temperaturesof about 50 C. to 60 C. when using the nitric acid-selenious acidsolution just described. With the solution at 57 C. treatment mustcontinue for a minimum time of about 10 minutes to produce the desiredresult. If the treatment is continued too long, a pitting of the basemetal results.

If the above selenious acid solution is made up with normal nitric acid(bath 2) instead of 4 normal then the temperature, for "selenidingnickel, can be reduced to about 45 C. for the same time of treatment.

These solutions are somewhat unstable due to the formation of nitrousacid. This can be overcome by the addition of sodium nitrite, in anamount of about 30 grams per liter, to the solution to form (bath 3). Atthe same time the temperature during the seleniding operation can bereduced to about 29 C. for the ten minutes of treatment.

If instead of 30 grams 60 grams of sodium nitrite is added to the 4normal HNOa solution (bath 3A) two minutes treatment at about 29 C. issufllcient for producing a film to which the selenium will firmlyadhere.

In the foregoing example I have illustrated ways of practicing myinvention for "selenidin8 nickel prior to the application of metallicselenium coatings thereto. The same process is used when iron is to bepretreated prior to coating with selenium. Iron surfaces ordinarilyrequire a shorter dipping time than do nickel surfaces. As examples, itwas found that iron when treated with (bath 1) required only two minutesat 29 C. and with (bath 2) only three minutes at 22 C.

Many other metals can be provided with a coating of a metal-seleniumcompound in exactly the same way. For example, magnesiumaluminum alloys,such as that known as Dow- Metal, can be dipped in aqueous solutionscontaining selenious acid, thereafter washed and dried, and finallycoated with metallic selenium in any convenient way. With this alloy aselenium compound having a gray color is formed but the actual chemicalcomposition of such compound is not clearly understood. Dow-Metal isdestroyed by acids so for this metal the nitric acid is omitted. Intests it was found that a 60% S802 solution produced a satisfactory filmwith a two minute treatment at 21 C. and a 10% S802 solution produced asatisfactory coating in three minutes at 21 C.

Tests alsoindicate that the same beneficial effects can be obtained withother metals, for example, the results of treating zinc with three typesof solution to give satisfactory films are I Tempera- Bath Metal Time mm140 Minutes C. mseori-"i norm HNOQ Zn 1 21 %%8e0|+2 norm BNO; Zn 5 21%NaSe04+2 norm BN0. Zn 2 21 It was found that while selenious acid(SeOa) constituents which will react with the metal to form metal ionsthereof so that these positive ions can react with divalent negativeselenium ions to form a metal selenium compound. Copper, nickel, iron,zinc, tin, magnesium-aluminum alloys, steels of various kinds, and othermetals can be selenided as described above.

Before seleniding the surface of the metal it is advantageous to firstthoroughly clean it to free it of any solid particles, grease, rust, andthe like. While this canbe accomplished in any way, I find itadvantageous to first wipe the metal with a cloth to free it of anysolid particles, dust, etc., then degrease the metal with any suitablesolvent for oils and fats, such as trichlorethylene, ethylenedichloride, and the like, then subject the metal to an -electro-cleaningoperation in ways known to the art. Finally the thus treated metal canbe washed in hot and cold water and pickled in hydrochloric acidsolutions, for example, to remove any rust and oxidation products. Metalsurfaces so treated and finally washed free of acid are quite readilywetted by selenium so that homogeneous films of metallic selenium can beapplied thereto but the selenium does not adhere very strongly. But byselenium treating the surface of the metal prior to the application ofthe selenium, as described above, I am able to obtain selenium-coatedsurfaces wherein the selenium metal has greatly improved adherences.

Having thus described my invention, what I claim is:

1. In the coating of metal surfaces for the production of seleniumcoated plates the steps which comprise treating the metal surface with asolution containing a form of selenium to form a metal-selenium compoundthereon and thereafter coating the treated surface with selenium.

2. The process as in claim 1 wherein the surface to be coated is nickel.

3. The process as in claim 1 wherein the surface to be coated is zinc.

4. The process as in claim 1 wherein the surface to be treated is amagnesium-aluminum alloy.

5. The process of coating metal surfaces with selenium which comprisesthe steps of treating the surface with an aqueous solution containingnitric and selenious acid to form a metal selenium compound on thesurface and thereafter coating the treated surface with selenium.

6. The process as in claim 5 wherein the surface to be coated 1s nickel.

7. The process as in claim 5 wherein the surface to be coated is zinc.

8. A selenium-coated metal plate having a selenium compound of the metalformed by sub- Jecting the surface of metal to an aqueous solu- ,tion ofselenious acid underlying the selenium coating.

9. Selenium-coated nickel having a nickel-selenium compound formed bysubjecting the surface of the nickel to an aqueous solution of seleniousacid underlying the selenium coating.

10. A process of treating galvanized iron surfaces with selenious acidto form a zinc selenium compound thereon.

ARTHUR VON I-IIPPEL.

